[PATCH 2/3] drm/dp/mst: Calculate total link bandwidth instead of hardcoding it

[Ville Syrjälä]

On Thu, Nov 17, 2016 at 06:03:47PM -0800, Dhinakaran Pandiyan wrote:
> The total or the nominal link bandwidth, which we save in terms of PBN, is
> a factor of link rate and lane count. But, currently we hardcode it to
> 2560 PBN. This results in incorrect computation of total slots.
> 
> E.g, 2 lane HBR2 configuration and 4k at 60Hz, 24bpp mode
>   nominal link bw = 1080 MBps = 1280PBN = 64 slots
>   required bw 533.25 MHz*3 = 1599.75 MBps or 1896 PBN
>      with +0.6% margin = 1907.376 PBN = 96 slots
>   This is greater than the max. possible value of 64 slots. But, we
>   incorrectly compute available slots as 2560 PBN = 128 slots and don't
>   return error.
> 
> So, let's fix this by calculating the total link bandwidth as
> link bw (PBN) = BW per time slot(PBN) * max. time slots , where max. time
> slots is 64
> 
> Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 04e4571..26dfd99 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -2038,9 +2038,8 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
>  			ret = -EINVAL;
>  			goto out_unlock;
>  		}
> -
> -		mgr->total_pbn = 2560;
> -		mgr->total_slots = DIV_ROUND_UP(mgr->total_pbn, mgr->pbn_div);
> +		mgr->total_pbn = 64 * mgr->pbn_div;

Do we actually have a use in mind for total_pbn? It seems unused now.

> +		mgr->total_slots = 64;

Same for total_slots.

>  		mgr->avail_slots = mgr->total_slots;

So avail_slots is all that's used. And shouldn't it actually start
out at 63 instead of 64 on account of the MTP header always taking
up one slot?

>  
>  		/* add initial branch device at LCT 1 */
> -- 
> 2.7.4

-- 
Ville Syrjälä
Intel OTC